You will ALWAYS want Teleport and Flash. Flash for obvious reasons, and Teleport because it helps you return to lane in the earl to mid game, as well as help your team push/team fight in the mid to late game. People will say "Run Ghost", but your object in the early game isn't to out damage your opponent like other mages. Your'e trying to wave clear faster so that they loose gold under their own turret and to get more gold using Spelltheif's Edge. Putting you slightly ahead and then getting back to lane faster because you have Teleport. This will help get items faster than your opponent when done right.

These two are your core items. Although you will want to buy Frostfang before you start building Lich Bane, you will only do that because Frostfang will help you continue to get more gold than your opponent. After that, start focusing Lich Bane. It will help you start farming with your Q better AND in the mid to late game it will help you take objectives faster/fight better.

I personally haven't ran into a mid laner I could not handle. AP Nasus is 100% skill match up, as long as you follow the general rule... don't try to do TOO much in the beginning. Get gold, push lane, buy items. Once the roaming begins, the lane phase won't match and your enemy won't be that intimating anymore (if he or she even was to begin with).